Step 1: First blank.
The sentence begins:
\[
\text{Nature is __________ and unchangeable...}
\]
The best adjective here is
relentless because it captures the ceaseless, unyielding, and intense quality of nature. “Harsh” is possible, but “relentless” better conveys the vast, powerful, and unchangeable aspect of nature.
Step 2: Second blank.
\[
\text{...and it is __________ as to whether...}
\]
Nature does not show concern or bias. The correct word is
indifferent, which implies neutrality and lack of partiality.
Step 3: Third blank.
\[
\text{...its hidden reasons and __________...}
\]
“Actions” fits logically, since nature’s actions (like storms, floods, seasons) may or may not be understandable to man. “Activities” is less idiomatic here.
Step 4: Fourth blank.
\[
\text{...are __________ to man or not.}
\]
The appropriate word is
understandable, meaning capable of being comprehended.
Step 5: Verify sentence.
\[
\text{Nature is relentless and unchangeable, and it is indifferent as to whether its hidden reasons and actions are understandable to man or not.}
\]
This is logical, meaningful, and grammatically correct.
\[
\boxed{\text{relentless, indifferent, actions, understandable}}
\]